Selective desensitizer



Patented Aug. 17, 1937 UNITED STATES PATENT OFFIQE.

SELECTIVE DE SENSITIZER No Drawing. Application July 25, 1934, SerialNo. 736,969

2 Claims.

This invention relates to a method of selectively limiting or reducingthe spectral sensitivity of gelatino-silver-halide photographicemulsions.

The usefulness of most photographic elements,

5 such as printing and enlarging papers, process plates and films, cinepositive films and X-ray films, depends on their sensitivity to theshorter Wave lengths of light (ultra violet to blue). In addition theypossess a very slight sensitivity to 10 the longer wave lengths(yellow-red) which is of no practical usefulness and is really adisadvantage. Yellow-red light of low intensity is most frequentlyemployed to illuminate dark rooms where the above mentionedphotographicproducts 15 are processed. The brightness of this light or converselythe length of exposure under it, is determined by the yellow-redsensitivity of the photographic product being handled. If the thresholdexposure is exceeded it will be found 2 on development that an over-alldensity has been given to the product which detracts from its quality.This is particularly noticeable in the high lights.

I have found a class of compounds, the purines, which, when added to anyof the above mentioned 5 emulsions, reduce the yellow-red sensitivitywithout affecting the sensitivity to the shorter wave lengths. Among thepreferred compounds I have found particularly effective in embodying myin- 3 vention are: purine, di-iodo-purine, uric acid,

adenine, caffeine, xanthine, guanine, hydroxycaffeine, l-methylxanthine, B-methyl xanthine, 7-methyl xanthine, theophylline,theobromine, para-xanthine, and hypo-xanthine. The purine 35 orsubstituted purine is added to the emulsion at any point during itsmanufacture, with any of its constituents, or just before coating on thesupport. It may be introduced in the form of a solution in water orwater miscible solvent or 40 dilute alkali hydroxide in any of thesesolvents, or in X-ray film it may be added to the emulsion top-coatingor anti-abrasion layer.

I have found the preferred amount of these compounds to be added rangesfrom 50 milligrams to 3 grams per kilogram of the dry constituents ofthe emulsion (gelatin and silver halide. I do not limit myself to theuse of a single compound but a mixture of two or more purine derivativesmay be employed without departing from the scope of my invention.

The following example does not limit but is illustrative of myinvention.

Erample.To one litre of silver halide emulsion is added with stirring 10cc. of an alcoholic solution containing 50 mgs. of theophylline (1:3dimethyl xanthine). It Will be found that films coated with thisemulsion can be exposed to the yellow-red safe lights of a photographicdark room for a longer period of time, or conversely, to a higher levelof illumination, than can films containing no theophylline in theemulsion layer, without the formation of an over-all density which isrevealed on development.

As many apparent and Widely different embodiments of this invention maybe made Without departing from the spirit thereof, it is to beunderstood that I do not limit myself to the foregoing examples ordescription except as indicated in the following claims.

I claim:

1. A gelatine-silver-halide emulsion of reduced yellow-red sensitivitycontaining purine in the approximate proportion of 50 milligrams to 3grams per kilogram of gelatine and silver halide in said emulsion.

2. A gelatine-silver-halide emulsion of re duced yellow-red sensitivitycontaining a purine in the approximate proportion of 50 milligrams to- 3grams per kilogram of gelatine and silver halide in said emulsion.

WALTER DEWEY BALDSIEFEN. 40

